Zirconium coating for electron discharge devices



Jan. 2, 1951 E. G. WIDELL 2,536,673

ZIRCONIUM COATING FOR ELECTRON DISCHARGE DEVICES Filed Feb. 25, 1948 INVENTOR EMIL E. WIDELL BY WI .4T ORNEY Patented Jan. 2, i951 ZIRCONIUM COATING FOR ELECTRON DISCHARGE DEVICES Emil G. Widell, Bloomfield, N. J., assignor to Radio Corporation of America, a corporation of Delaware Application February 25, 1948, Serial No. 10,735

Claims. 1

Elements comprising a base metal coated with zirconium have heretofore been employed in electron discharge devices for securing a desired heat dissipation and gettering action.

One of the dfficulties encountered in the use of zirconium as a coating resides in the readiness with which the zirconium alloys with the base metal at the elevated temperatures requred during fabrication and experienced during the normal use of the device in which it is used.

Heretofore, procedures for making electron discharge devices have required relatively low temperatures, which made it unnecessary to resort to any means for preventing alloying between the zirconium coating and the base metal. However, recent manufacturing techniques and power output requirements have indicated the importance of preventing such alloying action.

It is therefore an object of the invention to provide a coatin of zirconium on a base metal that will be free from substantial alloying with the base metal.

An additional object is to provide a coating of zirconium on an electrode of an electron discharge device that will not appreciably alloy with the basemetal of the electrode at the elevated temperatures to which it is necessary to subject the device.

A further object is to provide a zirconium coating on an element of an electron discharge device for conferring good heat dissipation and gettering action thereto, wherein the coating includes an agent which serves to prevent undesired alloying between the zirconium and the metal of said element.

Another object is to include in the zirconium coating an agent that is effective for preventing alloying of the base metal and the coating, and limiting the quantity of said agent in relation to the zirconium in such a manner that the amount of said agent in the coating is large enough to prevent substantial alloying with the base metal but insufficient to harm the heat disslpation and gettering action of the coating.

A still further object is to provide a coating including zirconium and zirconium oxide in predetermined quantity relations whereby the zirconium oxide effectively prevents substantial alloying between the zirconium and the base on which it is coated, and at the same time only very negligibly interferes with the heat radiating and gettering properties of the zirconium.

Further objects and advantages will appear as the description proceeds.

Zirconium metal in powdered form suitably carried by a binder and applied as a coatin to an electrode, such as the anode, of an electron discharge device and subsequently baked to remove the binder and partially sinter the particles of zirconium, exhibits desirable heat radiation properties because it presents a dark surface approaching the charatceristics of a black body, and good gettering action resulting from the porous mass which is capable of absorbing relatively large quantities of deleterious gases.

One of the difiiculties associated with the use of a zirconium coating on electrodes of electron discharge devices is the fact that the zirconium metal alloys with the base metal of the electrode at the elevated temperatures required for fabrication and subsequent use of the device. The resultant alloy is unsuitable either for good heat dissipation or gettering. Some means is therefore required to prevent such alloyin while preserving the heat radiating and gettering properties of the coating.

According to the invention, this means comprises zirconium oxide added to the powdered coating mixture in a quantity having a critical relationship to the quantity of the zirconium metal. It has been determined that an addition of zirconium oxide within a critical range of quantity values with respect to the zirconium metal not only effectively prevents substantial alloying between the zirconium and the base as will impair the desirable properties of the zirconium previously referred to. but permits a de sirable and limited alloying action between the coating and the base which is required for good adherence of the coating on the base.

According to the invention the mixture to be sprayed on a metal base to provide a coating of zirconium thereon, is prepared by adding to from one to nineteen parts of powdered zirconium hydride, or powdered zirconium metal, by weight, one part of powdered zirconium oxide. This mixture is suitably held by a binder and sprayed on the base metal to be coated in a conventional manner. The sprayed element is then heated to liberate the hydrogen and in the event the element constitutes a part of an electron discharge device, the heat applied is high enough to thoroughly outgas the part.

An iron base coated with a mixture containing one part of zirconium oxide to nineteen parts of zirconium hydride by weight, has successfully withstood temperatures as high as 1000 C. with-.

out resulting in alloying of the zirconium and the iron. However, when the iron is employed as an electrode, forexample, the anode of an electron discharge device, it is necessary to subject thecoated element to a temperature of 1050 C. to thoroughly outgas it. In this instance a mixture containing one part zirconium oxide to three parts zirconium hydride by weight was found satisfactory. Without any oxide the alloying action would take place at 900 C.

A molybdenum anode for a high power el tron discharge device, having a coating thereon comprising one part zirconium oxide, and three parts zirconium hydride by weight, has successfully withstood temperatures as high as 1740 C. without significant alloying between the molybdenum and the zirconium coating. In addition, the coating was firmly adherent and showed no tendency toward peeling and cracking.

Although the use of a coating mixture containing one part of zirconium oxide to three parts of zirconium hydride or powdered zirconium metal, by weight, was found to be sufficient for pre venting the alloyin of the zirconium and the metal base. which in the previous examples was made of iron and molybdenum, it is possible that other metals may require a larger percentage of zirconium oxide in the mixture than that indi- 1 cated for preventing the alloying action. However, it should be borne in mind, that an excessive amount of zirconium oxide in the mixture may adversely affect the heat radiating and gettering properties of the zirconium coating as well as the adherence of the coating to the metal base. Therefore, according t the invention, it has been determined that the maximum permissible amount of zirconium oxide in the mixture without appreciably harming the desirable characteristics of the coating referred to, is one part of zirconium oxide to one part of zirconium hydride by weight.

In accordance with the invention, it has also been determined that a lower limit exists with respect to the amount of zirconium oxide in the coating. This lower limit is one part of zirconium oxide to nineteen parts zirconium hydride or powdered zirconium metal, by weight. A lesser amount of zirconium oxide than this should not be used if an appreciable and undesired amount of alloying of the zirconium and the metal base is to be prevented.

It will be readily appreciated that this relatively wide range of quantity values applicable 'to the zirconium oxide content in the coating that is permitted by the invention, renders it practicable to operate zirconium coated elements of iron, molybdenum or other metals at much higher temperatures than those indicated herein without the objectionable alloying action referred to and without appreciably impairing the heat radiating and gettering characteristics of such elements.

In fabricating anodes for electron discharge devices in accordance with the invention, the

anode base metal may be supplied with a coating either of powdered zirconium metal or powdered zirconium hydride. If the hydride is used the anode may be either pro-baked in a vacuum to decompose the hydride before assembly in an electron discharge device, or the sprayed anode may be mounted in such device and the hydride may be decomposed during exhaust. Iron anodes are usually mounted in the device prior to decomposition of the hydride, while molybdenum anodes are usually pre-baked. With zirconium Oxide present in the coating material in the amounts indicated, the elevated temperatures required for the baking and exhaust have no harmful effects on the zirconium coating.

The spray mixture is prepared, for example, by first milling zirconium hydride powder received from commercial sources for a period of about 24 hours. The resultant pulverized material is then mixed with a critical amount of finely powdered electrical fused zirconium oxide, the amount of the latter bein determined by the standards described herein. The zirconium oxide and the zirconium hydride are then milled for an additional four hours to insure a thorough mixing together thereof. A plastic vehicle for the mixture may then be added. A variety of suchvehicles are known and are available inv the art. As a result of this mixing, the zirconium hydride particles are partially separated from each other and when sprayed on to the base metal, they are also partially separated from the base metal. This partial separation of the zirconium hydride particles from the base metal continues after baking and pernits only a limited sintering or alloying effect to occur between the resulting zirconium and base metal. which, when the portions of zirconium oxide specified herein are used, is just sufficient to cause the coating to adhere firmly to the metal base.

While I believe the effectiveness of the critical proportions of the zirconium oxide and zirconium hydride in the spray mixture in preventing appreciable alloying between the zirconium and the base metal, is due to the shielding effect produced by the zirconium oxide particles, with respect to the zirconium particles during the processing of the tube, whereby only a limited amount of zirconium is permitted to come in contact with the base metal, I do not wish to be limited to this explanation. Whatever the function of the zirconium oxide may be, I know that its presence in the coating mixture in the proportions indicated herein, has the very useful effect of preventing more alloying between the zirconium and the metal base than is required for securing a firm bond therebetween, while preserving the desirable characteristics of the coating with respect to good heat radiation and gettering.

Referring to the drawing, there is shown in the single figure thereof a fragmentary crosssectional view of an anode of an electron discharge device with a portion thereof greatly enlarged t illustrate the nature of the coating applied to the base metal. The base metal ll! of the anode is shown, provided with a coating II for increasing heat radiation therefrom and for absorbing deleterious gases within an envelope within which the anode may be used.

As more clearly shown in the large cutaway portion of the figure, the coating includes a plurality of discreet particles [2, shown in solid black, of zirconium metal, and a plurality of particles l3 of zirconium oxide, shown in white and disposed between the z'rconium particles. The relative amounts of zirconium and zirconium oxide particles observe the ratio range previously referred to herein.

It will be noted that the particles I3 surround one or a group of particles I2, and that a limited number of particles l2 of zirconium engage the surface of the base metal ll! of the anode. This limited number of zirconium particles is sufficient to form an adherent bond between the anode base II and the coating II when sintered or alloyed with the base l at the baking or exhaust temperatures required for the completion of the device. The presence of the particles l3 effectively prevents alloying of a larger number of particles It with the base l0 than are in actual contact therebetween due to the shielding action of the particles l3.

with the proportions of zirconium and zirconium oxide indicated, the surface ll of the coating will retain its good heat radiating properties and will have unimpaired gettering action and good adherence to the base l0.

Various modifications may be made in the invention without departing "from its spirit and scope as pointed out in the appended claims.

I claim:

1. An article comprising a structure having thereon a coating comprising a mixture of three parts by weight of zirconium and one part of zirconium oxide.

2. An article comprising a structure having thereon a coating comprising a mixture of from 1 to 19 parts by weight of zirconium metal and one part of zirconium oxide.

3. An electrode for an electron discharge device comprising a structure having thereon a coating of good heat dissipating. adhering and gettering characteristics, said coating comprising a mixture of from 1 to 19 parts of zirconium particles and one' part of zirconium oxide particles. said coating having a surface adjacent said structure consisting of a plurality of said zirconium particles separated by said zirconium oxide articles, some of said zirconium particles in said surface being sintered to said structure, whereby said coating is firmly adherent to said structure and is free from excessive alloying therewith.

4. An electrode for an electron discharge device comprising a structure having thereon a coating of a homogeneous body formed by a mixture of zirconium particles and a predetermined relative amount of zirconium oxide particles. groups of zirconium particles being surrounded by a plurality of said zirconium oxide particles. 9. surface of said body engaging said structure, said surface being fused to said structure at a plurality of locations displaced from said zirconium oxide particles. said body having another and free surface, said free surface including a plurality of zirconium metal particles for good heat dissipation and gettering action. 7 '5. An article comprising a" structure having thereon a coating comprising a mixture of zirconium and zirconium oxide particles, the amount of said zirconium particles being from to 94% by weight and the amount of said zirconium oxide particles being from 50% to 6% of said mixture.

EMIL G. WIDELL.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,958,967 Kniepen May 15, 1934 2,029,144 Wiegand Jan. 28, 1936 2,368,060 Wooten Jan. 23; 1945 2,417,460 Eitel Mar. 18, 1947 FOREIGN PATENTS Number Country Date 482,022 Great Britain Mar. 22. 1938 

3. AN ELECTRODE FOR AN ELECTRON DISCHARGE DEVICE COMPRISING A STRUCTURE HAVING THEREON A COATING OF GOOD HEAT DISSIPATING, ADHERING AND GETTERING CHARACTERISTICS, SAID COATING COMPRISING A MIXTURE OF FROM 1 TO 19 PARTS OF ZIRCONIUM PARTICLES AND ONE PART OF ZIRCONIUM OXIDE PARTICLES, SAID COATING HAVING A SURFACE ADJACENT SAID STRUCTURE CONSISTING OF A PLURALITY OF SAID ZIRCONIUM PARTICLES SEPARATED BY SAID ZIRCONIUM OXIDE PARTICLES, SOME OF SAID ZIRCONIUM PARTICLES IN SAID SURFACE BEING SINTERED TO SAID STRUCTURE, WHEREBY SAID COATING IS FIRMLY ADHERENT TO SAID STRUCTURE AND IS FREE FROM EXCESSIVE ALLOYING THEREWITH. 